Energy harvesting for wireless sensor operation and data transmission

ABSTRACT

A device for powering a load from an ambient source of energy is provided. The device includes an energy harvesting device for harvesting energy from the ambient source of energy wherein the rate energy is harvested from the ambient source of energy is below that required for directly powering the load. A storage device is connected to the energy harvesting device. The storage device receives electrical energy from the energy harvesting device and is for storing the electrical energy. A controller is connected to the storage device for monitoring the amount of electrical energy stored in the storage device and for switchably connecting the storage device to the load when the stored energy exceeds a first threshold. The system can be used for powering a sensor and for transmitting sensor data, such as tire pressure.

RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 10/379,223, incorporated herein by reference, and claims benefit ofprovisional patent application 60/362,432, filed Mar. 7, 2002,incorporated herein by reference and provisional patent application60/443,120, filed Jan. 28, 2003, incorporated herein by reference. Thispatent application is related to the following US patent applications:

-   Ser. No. 09/731,066, docket number 1024-034, filed Dec. 6, 2000,    incorporated herein by reference;-   Ser. No. 09/757,909, docket number 1024-035, filed Jan. 10, 2001,    incorporated herein by reference;-   Ser. No. 09/801,230, docket number 1024-036, filed Mar. 7, 2001,    incorporated herein by reference;-   Ser. No. 09/768,858, docket number 1024-037, filed Jan. 24, 2001,    incorporated herein by reference;-   Ser. No. 09/114,106, docket number 1024-041, filed Jul. 11, 1998,    incorporated herein by reference;-   Ser. No. 09/457,493, docket number 1024-045, filed Dec. 8, 1999,    incorporated herein by reference; and-   non-provisional patent application having docket number 115-004,    Ser. No. 10/379,224, filed the same day as this application,    incorporated herein by reference.

FIELD OF THE INVENTION

This invention generally relates to collecting and transmitting data.More particularly, it relates to a device for sensing, storing andtransmitting data. Even more particularly, it relates to a device thatcan that can be attached to a structure or live subject and that canharvest energy from its environment to power sensing, storing andtransmitting data about the structure or live subject.

BACKGROUND OF THE INVENTION

Several available devices convert mechanical energy in the localenvironment into electrical energy, including the Seiko “Kinetic” watchand mechanical wind-up radios. An article, “Energy Scavenging withShoe-Mounted Piezoelectrics,” by N. S. Shenck and J. A Paradisohttp://computer.org/micro/homepage/may_june/shenck/index.htm reports onsystems that capture energy from the user's environment to provideelectricity to wearable microelectronic devices without batteries. Theunobtrusive devices scavenge electricity from the forces exerted on ashoe during walking. The devices include a flexible piezoelectric foilstave to harness sole-bending energy and a reinforced piezoelectricdimorph to capture heel-strike energy. They also report on prototypedevelopment of radio frequency identification (RFID) tags which are selfpowered by a pair of sneakers.6 A recent report by Meniger et al.,entitled “Vibration-to-Energy Conversion”, discloses amicroelectromechanical system (MEMs) device for the conversion ofambient mechanical vibration into electrical energy through the use of avariable capacitorhttp://www.kric.ac.kr:8080/pubs/articles/proceedings/dac/313817/p48-meninger/p48-meninger.pdf.However, these MEMs systems only demonstrated 8 microwatts of power.Transmission of RF data over distances of 20 feet or more requiresmilliwatt power levels.

Low power sensors have been developed, as described on commonly assignedU.S. patent application Ser. No. 09/731,066, to Arms, that includes asensing unit for attaching to a structure or live subject for sensing aparameter of the structure or live subject. The sensing unit includes asensor, a data storage device, and a transmitting device. The datastorage device is for storing data from the sensor. Power is provided bya power supply such as a rechargeable battery or fuel cell. Therechargeable battery can be recharged by inductive coupling from anexternal control unit.

Over the past years, sensors, signal conditioners, processors, anddigital wireless radio frequency (RF) links have become smaller,consumed less power, and included higher levels of integration. The Ser.No. 09/731,066 application, for example, provides sensing, acquisition,storage, and reporting functions. Wireless networks coupled withintelligent sensors and distributed computing have enabled a newparadigm of machine monitoring.

A paper, “Wireless Inductive Robotic Inspection of Structures,” byEsser, et al, proceedings of the IASTED International Conference,Robotics and Applications 2000, Aug. 14-16, 2000, Honolulu, Hi.,describes an autonomous robotic structural inspection system capable ofremote powering and data collection from a network of embedded sensingnodes and providing remote data access via the internet. The system usesmicrominiature, multichannel, wireless programmable addressable sensingmodules to sample data from a variety of sensors. The nodes areinductively powered, eliminating the need for batteries orinterconnecting lead wires.

Wireless sensors have the advantage of eliminating the cost ofinstalling wiring. They also improve reliability by eliminatingconnector problems. However, wireless sensors still require system powerin order to operate. If power outages occur, critical data collected bythe sensors may be lost. In some cases, sensors may be hardwired to avehicle's power system. In other cases however, the need to hard wire toa power system defeats the advantages of wireless sensors, and this maybe unacceptable for many applications. Most prior wireless structuralmonitoring systems have therefore relied on continuous power supplied bybatteries. For example, in 1972, Weiss developed a battery poweredinductive strain measurement system, which measured and counted strainlevels for aircraft fatigue. Traditional batteries, however, becomedepleted and must be periodically replaced or recharged, adding anadditional maintenance task that must be performed. This is particularlya problem for monitors used for a condition based maintenance programsince it adds additional maintenance for the condition based monitoringsystem itself.

None of the systems for sensing changes in the environment havecollected available mechanical energy to provide the electricity forrunning the sensors, storing data from the sensor, or communicating thedata externally. Thus, a better system for powering sensors and storagedevices, and for transmitting data gathered by sensors is needed, andthis solution is provided by the following invention.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a schemefor powering a wireless sensor system with a source of energy thatavoids the need to provide, replace or recharge batteries; It is afurther object of the present invention to provide a scheme for poweringa load with energy harvested from an ambient source of energy in thelocal area, wherein the rate at which energy is harvested from theambient source of energy is below that required for directly poweringthe load.

It is a further object of the present invention to provide a scheme forpowering a sensor system with ambient mechanical energy collected fromthe environment of the sensor system or with energy obtained fromambient magnetic field coupled energy;

It is a further object of the present invention to provide a network ofsensor systems in which sensors on the network are powered with ambientenergy collected from the environment;

It is a further object of the present invention to provide a device formonitoring the health of a machine or another system in which themonitoring device is powered with ambient energy harvested from theenvironment.

It is a further object of the present invention to provide a sensingsystem that includes a sensor that is read with electrical energyobtained from the harvested mechanical energy;

It is a further object of the present invention to provide a wirelesstransmitter connected to receive and transmit information obtained bythe sensor, in which the wireless transmitter is powered with electricalenergy obtained from the harvested mechanical energy;

It is a feature of the present invention that a sensing system includesa component for harvesting ambient mechanical or magnetic energy andconverts this energy into electrical energy;

It is a feature of the present invention to provide a data collectiondevice that is powered by the electrical energy obtained from theharvested mechanical energy;

It is an advantage of the present invention that the data collectiondevice can provide information about the environment using energyharvested from the environment; and

It is a further advantage of the present invention that the datacollection device does not itself require maintenance for replacing orrecharging batteries.

These and other objects, features, and advantages of the invention areaccomplished by a device for powering a load from an ambient source ofenergy. The device comprises an energy harvesting device for harvestingenergy from the ambient source of energy wherein the rate energy isharvested from the ambient source of energy is below that required fordirectly powering the load. A storage device is connected to the energyharvesting device. The storage device receives electrical energy fromthe energy harvesting device and is for storing the electrical energy. Acontroller is connected to the storage device is for monitoring theamount of electrical energy stored in the storage device and forswitchably connecting the storage device to the load when the storedenergy exceeds a first threshold.

Another aspect of the invention is an energy harvesting systemcomprising a piezoelectric transducer and a rectifier. The system alsoincludes a reactive device having a high impedance approximatelymatching impedance of the piezoelectric transducer at its operatingfrequency for efficiently transferring energy from the piezoelectrictransducer to the reactive device. The system also includes a lowimpedance high capacity storage device. The system also includes acircuit for monitoring voltage across the reactive device, and fortransferring the charge from the reactive device to the low impedancehigh capacity storage device when the voltage across the reactive devicereaches a specified voltage value.

Another aspect of the invention is a device for sensing temperature orpressure in a tire. The device includes a tire and a PZT mounted on thetire. The device also includes a circuit for harvesting energy from thePZT, wherein the circuit comprises an element having an impedanceapproximately matching impedance of the PZT. The device also includes asensing module connected to the circuit, the sensing module including asensor and a circuit for wirelessly transmitting sensor information.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following detailed description ofthe invention, as illustrated in the accompanying drawings, in which:

FIG. 1 a is a perspective view of an energy harvesting addressablewireless sensing node of the present invention mounted on a machine orstructure;

FIG. 1 b is a perspective view of components within the energyharvesting addressable wireless sensing node;

FIG. 1 c is a perspective view of the energy harvesting cantilever shownin FIG. 1 b with variable mass for tuning to a vibration frequency ofthe machine or structure;

FIG. 1 d is a schematic diagram of a base station for receiving saidwirelessly transmitted information;

FIG. 2 is an alternative embodiment in which a large sheet of PZT fiberis embedded in material, such as a hull of ship so vibration or strainenergy transmitted through the hull can be harvested;

FIG. 3 a, 3 b are block diagrams of one embodiment of an energyharvesting addressable wireless sensing node of the present invention inwhich energy is harvested by a PZT;

FIG. 4 is a block diagram of an alternate embodiment of an energyharvesting addressable wireless sensing node of the present invention inwhich energy is harvested from a power transmission line;

FIG. 5 is a block diagram of the wireless sensing module shown in FIGS.3 a, 3 b;

FIG. 6 a is a timing diagram of voltage across capacitor C2 of FIG. 11;

FIG. 6 b is a timing diagram of voltage across capacitor C1 of FIG. 11;

FIG. 6 c is a timing diagram of voltage across the transmitter of FIG.11 showing how charge gradually stored in long term storage capacitor C1is used to briefly power the transmitter or transceiver;

FIG. 7 is a cross sectional view of a tire having an energy harvestingdevice of the present invention to power transmitting pressure andtemperature sense data;

FIG. 8 is a schematic diagram showing a receiver mounted in a vehiclethat receives signals indicating tire sensor data transmitted by each ofthe tires on the vehicle;

FIG. 9 is a diagram showing data from an experimental test showing thatthe PZT provided the same low current output as load resistance wasvaried from 100 ohms to 50 Kohms;

FIG. 10 is a diagram showing data from the experimental test showingthat the optimum load impedance, that delivers maximum power, was foundto be about 500 Kohms;

FIG. 11 a is a block diagram of an improved embodiment of an energyharvesting addressable wireless sensing node of the present inventionhaving an additional stage of charge storage, monitoring, switching, andimpedance conversion between the rectifier and the controller of FIG. 3a;

FIG. 11 b is a schematic diagram showing more detail than the blockdiagram of FIG. 1 a; and

FIG. 12 is a schematic diagram showing a wireless web enabled sensornetwork (WWSN) system that requires very little power.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors recognized that substantial efficiency incollecting, storing, and transmitting data from wireless sensors couldbe provided by harvesting energy from the environment.

This invention is aimed at developing a new class of sensing systemsthat can wirelessly report data without the need for maintaining orreplacing batteries. Instead, the sensing systems rely on harvestingvibration, strain energy, or magnetic coupled energy from the localenvironment for conversion to electrical power for storage and use tocollect, store, or transmit data by the sensing system. Thus, machines,structures, and live subjects can be monitored without the need forreplacing or recharging batteries or for a battery maintenance schedule.Truly smart structures and machines will thus be able to autonomouslyreport their condition throughout their operating life without themechanism used for reporting the data itself requiring maintenance. Thesystem can be used to run and communicate with actuators as well assensors.

One important use of the present invention is to improve traditionalcondition based maintenance. Condition based maintenance provides a moreaccurate and cost effective maintenance program for equipment orstructures. The present invention reduces unnecessary preventivemaintenance for the devices used to monitor. In addition to providingfor wireless communication without batteries, the present inventionprovides the components necessary to realize the potential benefits ofcondition based monitoring, including information acquisition, analysis,storage, and reporting technologies that substantially lower powerrequirements, making energy harvesting for condition based maintenance arealistic source of energy.

An illustration of condition based maintenance and another important usefor the present invention is aboard ships where batteryless sensingsystems may be used for wirelessly monitoring oil debris or oilcondition, tank & hull corrosion, combustion pressure,water-lubricated-bearing wear, and machine condition. The invention canalso be used for integrated, hierarchical machinery diagnostics &prognostics; machinery diagnostics & prognostics; open systemsarchitecture condition based maintenance; human—computer interfacecondition based maintenance; and diagnostic of insulation, such as wireand windings. The invention can also be used on land vehicles oraircraft for purposes such as to monitor and report tire temperature andpressure. In each case mechanical energy, such as the energy ofvibration of the vehicle, can be used to power the sensor and itsstorage or communications apparatus.

Batteries, and the additional maintenance burden for replacing orrecharging batteries, are avoided in the present invention by providingwireless sensing network systems which can harvest energy from the localenvironment to provide the power needed for their own operation.

Numerous sources of ambient energy can be exploited for energyharvesting, including solar, wind, thermoelectric, water/wave/tide,rotation, strain, and vibration. For shipboard monitoring applicationsbelow deck and for monitoring tire pressure and temperature, mechanicalenergy harvesting devices, such as those that harvest strain orvibrational energy are preferred. In Navy applications, strain energywould be available on engine mounts, ship hull sections, and structuralsupport elements. Vibrational energy would be available on dieselturbine engine components, propeller shaft drive elements, and othermachinery and equipment. This energy could be harvested usingelectromagnetic devices (coil with permanent magnet), Weigand effectdevices, and piezoelectric transducer (PZT) materials. Of these, the PZTmaterials hold the most promise.

Recent developments in single crystal PZT have led to significantimprovements in the mechanical-to-electrical conversion coefficients(coupling coefficients), from 60% efficiency to 90% efficiency. Singlecrystals also exhibit higher operating strain capabilities thanconventional PZT materials (0.2% vs. 1.4%). These materials areavailable through TRS Ceramics (State College, Pa.http://trsceramics.com/Single_Crystal.pdf).

Furthermore, PZT fibers have recently been made commercially availableat low cost for active damping of sporting equipment, such as baseballbats, tennis rackets, and skis (Advanced Cerametrics, Lambertville,N.J., www.advancedcerametrics.com/piezo_fiber.html). These fibers may bedirectly bonded to a straining element or structure to generateelectrical energy that can be harvested. Major advantages of these fiberpiezoelectric materials is that they can tolerate the loss of manyindividual fibers in a bundle and still function well. Since they are inmass production, they may be obtained readily and at relatively lowcost. Because of these advantages the present invention describes theuse of these PZT materials for energy harvesting wireless sensornetworks. However, other devices and other sources of ambient energy canalso be used.

The present inventors have used single crystal and PZT fibers to createworking energy harvesting prototypes that provide sufficient energy topower StrainLink wireless sensor transmitters available fromMicroStrain, Inc. (StrainLink, http://www.microstrain.com/slink.html).

Energy harvesting addressable wireless sensing node 18 can be attachedto machine or structure 19 that is subject to vibration, as shown inFIG. 1 a. In one embodiment, PZT 20 is mounted to cantilever 22 whichcan be tuned with variable mass 24, as shown in FIGS. 1 b and 1 c, toprovide a device resonance frequency close to the vibration frequency ofmachine or structure 19, thereby optimizing energy harvesting. PZT 20can be either a crystal or a fiber. Cantilever 22 is mounted on PC board25 in enclosure 26.

Alternatively, a large sheet of PZT fiber 27 can be embedded in materialof hull 28 of ship 30 so vibration or strain energy transmitted throughhull 28 can be harvested, as shown in FIG. 2. Large sheets of PZT fiber27 are preferred because tuning is not readily available to harvest thestrain energy. A structure, such as hull 28 or the deck of a bridgebends only a limited amount, and the bending cannot be tuned as canflexural element by adjusting mass so as to take advantage of resonancefrequency to harvest more of the energy.

In the mechanical vibration embodiment, the source of mechanical energy,such as machine or structure 19, is converted to electrical energy inenergy harvesting addressable wireless sensing node 18′, which includesa miniature electric generator, such as PZT 20, as shown in FIG. 3 a. Aminiature electric generator can also be provided with a coil and magnetin relative rotational motion, as for example, would be available in thevicinity of spinning machinery or wheels.

Electrical power generated in PZT 20 is rectified in rectifier 40,stored in electrical storage device 42, and once sufficient energy hasbeen stored, is provided to a load, such as wireless sensing module 44,by means of controller 46.

In one experiment, a single crystal PZT 20 was connected to the circuitshown in FIGS. 3 a, 3 b, while vibration was applied to PZT 20. With aDC voltmeter across storage capacitor 42, upwards of 20 volts wasmeasured across the capacitor with approximately 0.08 inch deflection ofthe PZT 20 at a 50 Hz rate.

PZT 20 is inherently a high impedance device which provides a nearlyconstant current, so the storage capacitor charges linearly with time.Thus, the time for storage capacitor 42 to charge is found from T=CV/Iwhere C=capacitance value, V=voltage charged to, and I=the chargingcurrent.

The Microstrain StrainLink transmitter is also a constant current load,so calculating the discharge uses the same formula. A 47 uF cap chargedto 16 volts will supply 2.8 mA for 268 mS discharging to zero volts. Alow power StrainLink transmitter can be connected as load 44 in thecircuit of FIG. 3 a, 3 b and can run for 224 mS before reaching thereset voltage of 2.63 volts. This is enough time to transmit data fromseveral sensors. Obviously a bigger storage capacitance would supply alonger operating time as would any reduction in load current presentedby the transmitter. However, a longer time would be needed to charge alarger capacitor. Furthermore, the practicality of such a system isdependant on the continued availability of vibration input energy. Thus,the present device is ideally suited to applications where ambientvibration is continuous for long periods to provide for theself-discharge rate of storage capacitor 42, to provide power consumedby the circuit used to monitor charge and switch on the load, as well asto power the load.

In an alternative embodiment PZT 20 device could be replaced with coilwinding 47 a that is closely coupled to a power transmission line 47 bthat would allow energy in the magnetic field around the transmissionline to be harvested, as shown in FIG. 4. Such a configuration could beused with thermocouples 47 c to measure the temperature of transmissionline 47 b and detect an overheated condition in transmission line 47 b.As with the PZT embodiment, the frequency of transmissions isproportional to current in the transmission line 47 b.

Wireless sensing module 44 includes microcontroller or microprocessor48, which controls provision of power to A/D converter 50, sensors 52,non-volatile memory 54, and RF transmitter 56, as shown in FIG. 5.Sensors can include such sensors as a temperature sensor, a straingauge, a pressure sensor, a magnetic field sensor, an accelerometer, ora DVRT. By selectively providing power to or withholding power fromthese devices microcontroller 48 can achieve substantial energy savings.Microcontroller 48 also controls flow of data from A/D converter 50,from sensors 52, to and from nonvolatile memory 54 and to RF transmitter56. A transceiver can be provided instead of RF transmitter 56 to enabletwo way communication, all powered by ambient vibrational energy.

The strain or vibrational energy 57 from the ambient environment isprovided to PZT transducer 20 mounted on a machine, structure, or livesubject, as shown in block diagram form in FIG. 3 a and in schematicform in FIG. 3 b. As indicated above, electrical output of PZT 20 isrectified in rectifier 40. DC output of rectifier 40 charges storagecapacitor 42. Controller 46 monitors charge stored on storage capacitor42, and when sufficient, provides Vcc power to wireless sensing module44 for transmitting sensor data through antenna 68 to receiver 69 a onbase station 69 b (FIG. 1 d). Receiver 69 a can be a transceiver.Controller 46 includes monitoring device 70, and switch Q1, which isformed of MOSFET transistor 72. When voltage across capacitor 42 issufficient, monitoring device 70 turns on to provide Vcc to wirelesssensing module 44. To reduce leakage and ensure that wireless sensingmodule 44 remains fully off and does not load storage capacitor 42 whenvoltage across storage capacitor 42 is below a threshold, transistor 72is provided. When transistor 72 turns on, ground connection fromwireless sensing module 44 is established.

Transistor 72 is needed because when voltage provided by storagecapacitor 42 is too low, monitoring device 46 cannot provide its outputin a known state. Monitoring device 46 may turn on falsely and load downstorage device 42, preventing it from ever charging up. Monitoringdevice 46 is not itself a reliable switch unless supply voltage is abovea threshold. To provide for operation in the regime when supply voltageis below that threshold, switch 72 is provided to ensure that wirelesssensing module 44 remains fully off. Switch 72 connected betweenwireless sensing module 44 and ground and to has a single threshold.

In operation in one embodiment, monitoring device 70 becomes valid at1.8 volts. Switch Q1 transistor 72 turns on at 2.0 V, enabling wirelesssensing module 44 when storage capacitor 42 has sufficient charge tooperate monitoring device 70 properly and can hold it off. Finally, whenvoltage at storage capacitor 42 reaches 6.3 V monitoring device 70 turnson and transfers charge from storage capacitor 42 to power wirelesssensing module 44 for a brief period, until voltage discharges back to2.9 volts, at which point monitoring device 70 turns off furthertransfer, and monitoring device 70 therefore continues to be in a validstate for subsequent operation, well above the 1.8 volts level neededfor proper operation in a determinate state.

Thus, when sufficient charge is provided to storage capacitor 42 toprovide a voltage equal to a higher threshold, monitoring device 70turns on and connects wireless sensing module 44 to storage device 42.This discharges storage device 42 down to a lower threshold voltage atwhich point monitoring device 70 turns off, disconnecting wirelesssensing module 44 from storage device 62. Storage device 42 can thenrecharge from energy supplied from PZT 20. However, if storage device 42fully discharges, or if potential across storage device 42 falls belowthe lower threshold then monitoring device 70 may not be sufficientlypowered to operate correctly. It may not fully disconnect wirelesssensing module 44 from storage device 42, and thus, wireless sensingmodule 44 may continue to load storage device 42, preventing it fromever recharging. To prevent this possibility, switch 72 is providedwhich disconnects wireless sensing module 44 from ground when potentialacross storage capacitor 42 falls somewhat below the lower threshold.

The present inventors found that impedance mismatch between PZT 20 andstorage capacitor 42 limits the amount of power that can be transferredfrom PZT 20 to storage capacitor 42. They recognized that energytransfer was limited by the fact that AC power generated by PZT 20 ispresented by the PZT at a very high impedance and at low frequency. Theyobserved that PZT 20 behaves as a constant current source, and that whenpiezoelectric elements are used to charge capacitors, such as storagecapacitor 42, charging is determined by the short circuit currentcapability of PZT 20. When storage capacitor 42 is charged from aconstant current source, such as PZT 20, storage capacitor 42 willcharge at a rate proportional to the current provided by the constantcurrent source. They further recognized that since the current availablefrom PZT 20 is low, a long time is needed to charge a large capacitance,such as storage capacitor 42, needed for powering devices such aswireless sensing module 44 or other transmitters. They recognized thefurther difficulty that the larger leakage current presented by largercapacitors may exceed the charge rate of the constant current providedby PZT 20.

The present inventors developed a circuit that efficiently convertspower from a high impedance current source, such as PZT 20, to a lowimpedance voltage source capable of charging a capacitor or batterystorage device. The inventors also developed an efficient way todetermine when enough power has been accumulated and applying thataccumulated power to a useful purpose.

In addition, the present inventors recognized that if the availablepower in the piezoelectric element were to be efficiently converted fromits low current and high impedance current source to a voltage source,the capacitor could be charged much faster than if the same capacitorwere charged directly from the short circuit current of thepiezoelectric element without this conversion. For example, if a voltageconverter can present a 500K load to the piezoelectric element,approximately matching its impedance, the element will deliver 17.5volts at 35 uA or 610 microwatts. If this power was then converted downto 100 ohms source impedance, even at 80% efficiency, the charge currentwould be more than 2.2 mA. By comparison, the output at the same levelof excitation of the piezoelectric element when loaded to 100 ohmswithout a converter, is 6 millivolts at 60 uA or 0.36 microwatts, about1,700 times less power.

The inventors of the present invention conducted empirical tests on asample of piezoelectric material in order to determine a viable topologyof conversion circuit. A test was performed on a sample of highlyflexible piezoelectric fiber. The sample was mounted in a 3 pointbending jig with a strain gauge attached to the material, and excited tothe same strain levels at three different frequencies. A decaderesistance substitution box was used to load the output in order todetermine the optimum load impedance for maximum power out of thematerial under these conditions. The same low current was measured asthe load resistance was varied from 100 ohms to 50 Kohms as shown inFIG. 9. The optimum load impedance, that delivers maximum power, wasfound to be about 500 Kohms, as shown in FIG. 10.

The present inventors found that further substantial improvement inenergy harvesting is available by adding an impedance converter circuitto the circuit of FIG. 3 a that provide better impedance matching to thehigh impedance of PZT 20, while still finally providing the largecapacitance needed to power wireless sensing module 44. The improvementto energy harvesting addressable wireless sensing node 18″, illustratedin block diagram form in FIG. 11 a and in a schematic diagram in FIG. 11b, provides an additional stage of charge storage, monitoring,switching, and impedance conversion between rectifier 40 and controller46 of FIG. 3 a. In addition to providing more efficient transfer ofenergy from PZT to long term storage device 42′, the improvement allowsa much larger capacitor or a battery to be used for that long termstorage 42′, and this enables more information transfer by wirelesssensing module 44.

PZT 20 connected to a source of mechanical energy, such as vibration orstrain 57, produces a high impedance AC voltage in accordance with thestrain or vibration 57 applied to PZT element 20. D1 and D2 (FIG. 11 b)form Schottky barrier rectifier bridge rectifier 40 that converts the ACvoltage from PZT 20 to DC. PZT 20 charges reactance element 78, such assmall capacitor C2 along curve 80 until a voltage equal to Vth3 isreached, as shown in FIG. 11 a and FIG. 6 a.

Voltage Vth3 is sufficient to turn on switch 2, transistor 82 whichconnects DC-DC converter 84 to ground, enabling DC-DC converter 84 toturn on and operate. When DC-DC converter 84 turns on, it converts thehigh voltage stored on small capacitor C2 to a low voltage at a lowimpedance for providing a small boost 86 to the charge on long termstorage capacitor 42′, capacitor C1, as shown along charging curve 88 inFIG. 6B. While long term storage capacitor C1 is charging, smallcapacitor C2 is discharging. The discharge of small capacitor C2, isshown along curve 90 in FIG. 6 a, providing the charge to boost thevoltage of long term storage capacitor C1 by the small step 86 shown inFIG. 6 b. Voltage scales are the same on FIGS. 6 a, 6 b, 6 c. Smallcapacitor C2 continues to discharge through DC-DC converter 84 untilvoltage on small capacitor C2 equals voltage on long term storagecapacitor C1. Thus, as long term storage capacitor C1 charges up, smallcapacitor C2 discharges less and less fully, as shown by the continuousincrease in the discharge voltage level 92 in FIG. 6A with each chargingand discharging cycle of small capacitor C2, while the charge level oflong term storage capacitor C1 continuously increases.

Although voltage on small capacitor C2 discharges, second switch 82remains on because of delay introduced by capacitor C3 in parallel withresistor R2 in voltage divider 94. Thus, DC-DC converter 84 remains onwhile voltage across capacitor C2 drops below Vth3 as shown in FIG. 6A.R4, R5 and second switch 82 form a switch that disables any conversionuntil enough voltage is present on C2 to convert. This switch thresholdis set to approximately 22 volts. DC-DC converter 84 is a high frequencystepdown DC to DC converter that has a typical quiescent current of 12uA and is capable of 80% efficiency even with small load current. Inthis embodiment, DC-DC converter 84, U2 is an LT1934-1 (LinearTechnology, Milpitas, Calif.). This converter was chosen due to its verylow quiescent current.

As also described for the circuit of FIGS. 3 a and 3 b and the circuitof FIGS. 11 a and 11 b, resistors R1, R2, R3, and comparator U1 formmonitoring device 70 a and also form voltage sensitive switch 70 b thatturns off connection to load 44 until enough charge has been accumulatedon storage capacitor 42, 42′ to run load 44. Load 44 can be multiplewireless sensing module 44, or an array of such modules, as shown inFIG. 11 b. Monitoring device 70 a/voltage sensitive switch 70 b is in anundefined state, however, until at least 1.8 volts is available on itsVcc pin 7, which is connected to storage device 42, 42′. To avoidproblems from this undefined state, MOSFET switch Q1 is provided todisconnect load 44 until voltage on storage device 42, 42′ has reached2.0 volts. R2 & R3 set the turn-on threshold V_(th2) of voltagesensitive switch 70 b to 6.3 volts. R1 provides hysteresis to comparatorU1 giving it two thresholds. Voltage sensitive switch 70 b now turns onwhen voltage on storage device 42. 42′ reaches the higher thresholdV_(th1) of 6.3 volts and stays on until the voltage on storage device 42discharges down to V_(th2) the lower threshold of 2.9 volts. Whenstorage device 42, 42′ reaches its higher threshold of 6.3 volts thereis enough charge available on storage device 42, 42′ to power load 44 tooperate for a brief period, for example, to transmit a burst of data.Load 44 may be a StrainLink transmitter or a data logging transceiver.

None of the charge provided to long term storage device 42′, is suppliedto wireless sensing module 44 until the voltage on long term storagedevice 42′ reaches the higher threshold, V_(th1), as shown in FIG. 6B.When voltage on long term storage device 42′, C1 reaches V_(th1),monitoring device 70 now turns on, as described herein above. Switch Q1(transistor 72) has already turned on before V_(th2) was reached, andcharge is now transferred from long term storage device 42′, C 1 tooperate wireless sensing module 44, as shown in FIGS. 6B and 6C. Voltageon long term storage device 42′, C1 discharges to V_(th2), about 2.9volts at which point monitoring device 70 turns off.

If voltage to switch Q1 declines too far, switch Q1 will turn off, andthis shuts off transmitter 44 until enough energy is accumulated instorage device 42′ to send another burst of data.

Multiple wireless sensing modules 44 or other devices can be provided ona network, each powered as described herein with energy harvested fromits environment. The multiple wireless sensing module 44 can transmit ondifferent frequencies or a randomization timer can be provided to add arandom amount of time after wake up to reduce probability of collisionsduring transmission. However, since the time for charging is likely todiffer from one wireless sensing module 44 to another, a randomizationtimer may not be needed. Each wireless sensing module 44 will transmitan address as well as data. Transceivers can be provided to eachwireless sensing module 44 to provide two way communication. Preferably,if a battery is used that is recharged from the environment, sufficientenergy will be available so it can wake up periodically to determine ifsomething is being transmitted to it. If not it can go back to sleepmode. If so, it can receive the transmission. All the members can bemanaged by a broadcast signal or each wireless sensing module 44 can beaddressed and programmed individually.

The present inventors have applied the energy harvesting system todesign a device for embedding in a tire by a tire manufacturer forharvesting energy and for monitoring parameters, such as tiretemperature and pressure on a vehicle and for transmitting the data, asshown in FIG. 7. The cross section of tire 100 shows the placement ofPZT 102, or several such PZT elements, on or within interior sidewall104 of tire 100 for gathering strain energy from flexing of tire 100 onrim 101 as the tire rotates. PZT 102 is connected to provide power toenergy harvesting addressable wireless sensing node 106 for transmittingdata from temperature and pressure sensors 108, such as Sensor Nor fromHorten, Norway, to report this tire data. Energy harvesting addressablewireless sensing node 106 can be programmed to provide it with a 128 bitaddress. With such a large address there are enough combinationspossible to allow every tire in the world to have a unique address.Thus, receiver 110 mounted in the vehicle can receive a signalindicating tire sensor data for each of the tires on the vehicle, asshown in FIG. 8. A display can provide the information to the operator.Alternatively, an alarm can signal when tire pressure or temperature isoutside specified limits. Interference from other vehicles can beavoided by displaying only data from tires having known addresses.

Local antennas 112 can be provided in each wheel well (not shown) andthe power output of energy harvesting addressable wireless sensing node106 can be adjusted to provide reliable communications within the wheelwell of the vehicle while avoiding interference with transmitters onadjacent vehicles.

Receiver 110, having antennas 112 positioned in each wheel well of thevehicle, can rapidly scan antennas 112 to determine the address andposition of each tire on the vehicle. Because of the scanning of theantennas, even if tires are rotated, the display can indicate thelocation of a tire having a problem. Most modern receivers have thecapability of accurately measuring received signal strength with fairlyhigh resolution. In the case of inner and outer wheels in a single wheelwell, these received signals can be qualified by received signalstrength indication to distinguish the tires in the wheel well, even ifthey are rotated. Thus, the tire further from the antenna will have theweaker signal strength. In addition, the serial numbers of each tirewould also be logged in the receiver flash memory to distinguish tireson the vehicle for feedback to a tire manufacturer.

One alternative to the tire position problem that does not requirescanning or multiple antennas, is to have a technician sequentially scana bar code on the tires at the time of tire installation on the vehicle,and communicate the tire position information to the automotivecommunications (CAN) bus or other communications bus within the vehicle,or even directly to the receiver. The position information is providedusing a different protocol than the information tires are sending sothis information can remain stored in the receiver while other dataabout the tire changes with each reading. In this way one receiveantenna could receive data and an identification code from all tires onthe vehicle, and the stored table linking identification and tireposition can be used to communicate the position, temperature, andpressure of each tire. Interference from transmitters on adjacentvehicles is avoided since they would not have known identificationcodes.

The present inventors have also found ways to reduce power consumptionas well as to provide power from energy harvesting. They recognized thatpower consumed by all of the system's components (sensor, conditioner,processor, data storage, and data transmission) must be compatible withthe amount of energy harvested. Minimizing the power required to collectand transmit data correspondingly reduces the demand on the powersource. Therefore, the present inventors recognized, minimizing powerconsumption is as important a goal as maximizing power generation.

The present inventors have developed and marketed sensors that requirevery little power. For example, they have previously reported onmicro-miniature differential variable reluctance transducers (DVRT's)capable of completely passive (i.e., no power) peak strain detection.These sensors can be embedded in a material and will continuouslymonitor for the existence of a damaging strain state. By providing ahermetic seal the sensors can withstand harsh environmental conditions(moisture, salt, and vibration). The sensors can be reset remotely usingshape memory alloys and (remotely applied) magnetic field energy, asdescribed in a copending patent application Ser. No. 09/757,909, docketnumber 1024-035, incorporated herein by reference. The present inventorshave also recently developed totally passive strain accumulationsensors, which can be used to monitor fatigue. Furthermore, they havedemonstrated novel radio frequency identification (RFID) circuits withthe capability of interrogating these sensors in under 50 microsecondsusing less than 5 microamperes of current. Thus, although small amountsof energy may be available from energy harvesting, the energy socollected is enough to power sensors, electronics, and transmitters.

The present inventors have also developed wireless web enabled sensornetwork (WWSN) systems that require very little power. One strategy forminimizing power is demonstrated by the WWSN network architectureillustrated in FIG. 12. This is an ad hoc network that allows thousandsof multichannel, microprocessor controlled, uniquely addressed sensingnodes TX to communicate to a central, Ethernet enabled receiver RX withextensible markup language (XML) data output format(http://www.microstrain.com/WWSN.html). A time division multiple access(TDMA) technique is used to control communications. TDMA allows savingpower because the nodes can be in sleep mode most of the time.Individual nodes wake up at intervals determined by a randomizationtimer, and transmit bursts of data. By conserving power in this manner,a single lithium ion AA battery can be employed to report temperaturefrom five thermocouples every 30 minutes for a period of five years. TheXML data format has the advantage of allowing any user on the local areanetwork (LAN) to view data using a standard Internet browser, such asNetscape or Internet Explorer. Furthermore, a standard 802.11b wirelesslocal area network (WLAN) may be employed at the receiver(s) end inorder to boost range and to provide bi-directional communications anddigital data bridging from multiple local sensing networks that may bedistributed over a relatively large area (miles). Further informationabout a wireless sensor network system developed by the presentinventors is in patent application docket number 115-004, incorporatedherein by reference.

Another strategy for creating low power wireless sensor networks isdemonstrated by MicroStrain's Data Logging Transceiver network(http://www.microstrain.com/DataLoggingTransceiver.html) as described incopending U.S. patent application Ser. No. 09/731,066, docket number1024-034, incorporated herein by reference. This system employsaddressable sensing nodes which incorporate data logging capabilities,and a bi-directional RF transceiver communications links. A central hostorchestrates sample triggering and high speed logging to each node or toall nodes. Data may be processed locally (such as frequency analysis)then uploaded when polled from the central host. By providing eachsensor node with a 16 bit address, as many as 65,000 multichannel nodesmay be hosted by a single computer. Since each node only transmits datawhen specifically requested, the power usage can be carefully managed bythe central host.

For further energy savings, only limited data collected by sensors maybe transmitted. For example, minimum, maximum and average data can betransmitted to reduce the amount of data transmitted and to thereby saveenergy. Standard deviation can also be locally calculated andtransmitted, saving transmission time and energy.

For sensors detecting information where a band of frequencies ismeasured, such as measurements of a vibrating source with anaccelerometer, a fast Fourier transform can be locally calculated andonly the frequencies of vibration and the magnitude of vibration need betransmitted, rather than the entire waveform, to reduce the amount ofinformation transmitted and to save energy.

The present inventors provided improved designs of each element of theentire measurement system, including the: vibrating/straining structure,piezo harvesting circuit, sensing circuit, microprocessor, on boardmemory, sensors, and RF data transmitter/transceiver to provide a systemthat operated with low power. The present inventors then built aprototype that both improved on the performance of energy harvestingdevices and that reduced the energy consumption of each element of themeasurement system, including the vibrating/straining structure, piezoharvesting circuit, sensing circuit, microprocessor, on board memory,sensors, and RF data transmitter/transceiver, as shown in FIGS. 3 a, 3b, 4 and 5.

A demonstration energy harvesting circuit was built using a PZT fiber asits input, as shown in FIGS. 3 a, 3 b. The PZT device generates avoltage that is rectified by low forward drop diodes. This rectifiedvoltage is used to charge a storage capacitor. The transfer is purely afunction of the short circuit current of the piezoelectric structure,minus the loss of the rectifier stage, the self discharge of the storagecapacitor, and any leakage current in the switch in its ‘off’ state. Thebehavior of this configuration is similar to charging a capacitor from aconstant current source. The time required to charge the capacitor isinversely proportional to the amplitude of the strain or vibrationapplied to the PZT element at a given frequency of strain, and alsoproportional to the frequency of strain at a given amplitude. Once thevoltage sensing switch detects that enough charge is stored on thecapacitor, the load is connected to the storage capacitor. The load inthis demonstration circuit is a MicroStrain Strainlink RF sensormicrotransmitter. (MicroStrain, Inc. Williston, Vt.http://www.microstrain.com/slink.html) StrainLink is a multichannel,digital wireless transmitter system which allows direct sensor inputsfrom five pseudo differential (single ended) or three true differentialchannels. StrainLink features on-board memory, with user programmabledigital filter, gain, and sample rates and with built-in error checkingof pulse code modulated (PCM) data. Once programmed, these settingsreside in the transmitter's non-volatile memory, which will retain dataeven if power is removed. The StrainLink transmitter is compatible withnumerous sensor types including thermocouples, strain gauges, pressuresensors, magnetic field sensors and many others. The transmitter cantransmit frequency shift keyed (FSK) digital sensor data w/checksumbytes as far as ⅓ mile on just 13 mA of transmit power supply current.During testing, the transmitter operated for approximately 250 mS on thepower stored in the charged capacitor. This was ample time for theStrainLink to acquire data from a sensor and transmit multiple redundantdata packets containing the sensed data.

Voltage sensing switch 70 b was implemented using a nano-powercomparator with a large amount of hysteresis. Some design difficultiesarise when using an electronic device to perform such switching tasks.Voltage sensitive switch 70 b itself needs to be powered from the sourceit is monitoring. When the available voltage is near zero the state ofswitch 70 b is indeterminate. This can present a problem when thecircuit is initially attempting to charge the capacitor from acompletely discharged state. In the demonstration circuit as built, theswitch defaults to ‘on’ until the supply voltage to its Vcc exceeds0.7V, then it will turn off until the intended turn-on voltage level isreached. The transmitter draws constant current, except when the supplyvoltage is below the transmitter's regulator threshold. In thiscondition the current increases slightly from the normal operatingcurrent of 11 mA to about 15 mA at less than 1 volt. Because of this,and the fact that the switch is ‘on’ below approximately 0.7 volts, asilicon diode with equal to or greater than 0.7 V forward drop was addedfrom the output of the switch to the transmitter power pin. This allowsthe storage capacitor voltage to charge to the point where the switch isactive before the transmitter starts drawing current. The settings forvoltage trip points were adjusted to 6.3V ‘on’ and 2.9V ‘off’.

In practice, the voltage sensing switch is still falsely ‘on’ at supplyvoltages of up to 1 volt, at which point the diode is already conductingpower into the load. Drawing current from the storage capacitor at thislow voltage slows the charging of the capacitor. This creates aproblematic “turn-on” zone where the capacitor is being drained at thesame time it is being charged. This makes it difficult for the system toinitially charge itself enough to begin operating properly. If enoughstrain energy is applied to the PZT element during initial systemstartup, then this turn-on zone is exceeded, and the system worksproperly.

The present inventors recognized that switching the positive rail e.g. a“high-side switch,” inherently requires some supply voltage to bepresent in order to properly turn the load “off.” This is not the casewith a “low-side switch,” or one in which the minus lead is switched toDC ground. FIGS. 3 a, 3 b, 11 a, 11 b illustrate an improvement to theswitch that will eliminate the turn-on zone problem. It employs both theexisting high side switch implemented with nanopower comparator V1, LTC150, and the addition of a low side switch in the DC return path of thepower source. The low side switch is implemented with an N channelenhancement mode MOSFET, such as first switch Q1, 72 that has a gateturn-on threshold higher than the minimum operating voltage of the highside switch. This combination eliminates the disadvantages of the highside switch and the difficulties with implementing the appropriateswitching function using only low side switch components.

High side voltage sensing switch V 1 may falsely turn on when storagecapacitor 42′ is charged to between 0.7 and 1.0 volts. No current willflow, however, until the supply voltage exceeds the Vgs voltage of thegate of MOSFET Q1, 72. The Vgs voltage is typically greater than 1.5volts even with so-called logic level MOSFETS that are optimized forfull saturation at logic level (5 volt) gate to source voltage. Once thecharge on capacitor 42′ has exceeded Vgs, the MOSFET will allow currentto pass, but by that point, the voltage sensing circuit will havesufficient supply power to function properly. These changes allow energyharvesting circuit 18′, 18″ to efficiently begin charging itself evenwhen it starts from a completely discharged state.

Efficiency of the energy storage element is an important factor inimplementing efficient designs because the energy may need to be storedfor significant time periods before it is used. In the demonstrationenergy harvesting system, an aluminum electrolytic capacitor wasutilized as the storage element. This is not an ideal choice since itsleakage loss is relatively high. In fact, it can be as much as ten timeshigher than that of the voltage sensing switch used to monitor thecapacitor voltage. To minimize this problem, alternative capacitortechnologies, such as tantalum electrolytic and ceramic, can be used.

No matter what capacitor technology is used, charge leakage is likely tobe a limiting factor in applications where long term storage of chargeis necessary. Batteries, can be used for long term energy storage device42, 42′, and have advantage of essentially zero charge leakage (<1%energy loss per year). Thin film batteries, such as those provided byInfinite Power Solutions, Littleton, Colo.www.infinitepowersolutions.com, offer advantage of being able to becharged and discharged in excess of 100,000 times. In addition, batterychemistry allows for a battery cell to be continuously charged whenpower is available, as supplied by the PZT. The battery cells have highenough peak energy delivery capability (10 mA pulsed power) to allow forshort bursts of RF communications.

Reduced power consumption is inherently beneficial to the performance ofsystems using harvested energy. A significant reduction in powerconsumption can be realized through the use of embedded software inmicrocontroller 48 that controls the power consumed by the sensors,signal conditioning, processing, and transmission components of theenergy harvesting wireless sensing systems (FIG. 5). By adjusting thetime these devices are on, for example, power consumed can be reduced.In addition embedded processor 48 can be programmed to process and storesensed information rather than immediately transmit, and thereby reducethe frequency of data transmission. Finally the power levels used for RFcommunications can be reduced by bringing a receiver closer to thesensor nodes. This can be accomplished by providing multiple receiversfor a sensor network, by bring an operator with a receiver closer, or byproviding a mobile robot that approaches sensors and reads their data,as more fully described in copending application docket number 115-004,incorporated herein by reference.

The most direct strategy to reduce the power consumed by the sensors andsignal conditioners is to use sensors that do not require power, such asthermocouples, piezoelectric strain gauges, and piezoelectricaccelerometers. For thermocouples, cold junction compensation can beprovided with a micropower solid state temperature sensor (NationalSemiconductor, Milpitas, Calif.) that typically consumes 20 microampscurrent at 3 volts DC, for a continuous power consumption of only 0.06milliwatts.

A second strategy is to employ sensors that do not need to transmit datafrequently, such as temperature and humidity sensors. There are severalvery low power humidity sensors, for example from Honeywell that can beemployed along with thermocouples or solid state temperature sensors toprovide periodic data updates. For these types of measurements, thereading changes slowly, so energy can be conserved by transmitting thedata infrequently.

A third strategy to minimize the power consumed by sensors 52 is topulse the power to sensors 52 and to time the reading of data from A/Dconverter 50 appropriately. With the sensor on only for a brief periodof time to achieve a stable reading and to obtain that reading forstorage or transmission, much energy can be saved. Microstrain hassuccessfully used this technique for powering and gathering data fromstrain gauges used in medical implants. The current, and therefore thepower, savings that can be realized are significant. For example, a 350ohm strain gauge bridge excited with 3 volts DC will consumeapproximately 8.6 milliamps. If powered continually, this represents apower drain of 25 milliwatts. By only providing the excitation voltageat periodic intervals and performing digital data conversion undermicroprocessor control, we have been able to reduce the sensorexcitation time to 200 microseconds. For applications where a straingauge reading is required every 100 milliseconds (10 Hz), the effectivepower drain is reduced by a factor of 500, (from 25 to only 0.05milliwatts).

Power reductions in the signal conditioning are also realized by usinghighly integrated circuits (IC), such as the AD7714 by Analog Devices(Norwood, Mass.), that use very low power and combine a programmablegain instrumentation amplifier (110 dB CMRR), multiplexer, and 22 bitsigma-delta analog to digital converter. The current consumed by theAD7714 is 200 microamps at 3 volts DC, or 0.6 milliwatts. The AD7714 canbe programmed to accept 3 full differential or five single ended sensorinputs. We have successfully employed this IC for use with foil andpiezoresistive strain gauges, thermocouples, temperature sensors, torquesensors, and load cells. With a rectifier, a differential amplifier andperiodic excitation we have successfully used these ICs with inductivedisplacement sensors.

The power consumed by the embedded processor can be reduced by using lowpower embedded microcontrollers, such as the PIC 16 series fromMicroChip Technologies (Chandler, Ariz.). Such embedded processorsinclude integrated instrumentation amplifiers to facilitate sensorconditioning, and integrated radio frequency (RF) oscillators tofacilitate wireless communications. By including more capability on theprocessor, component count and system complexity are reduced, and thereis a reduction in power consumed. Further reductions in powerconsumption are realized by placing the processor in “sleep mode” whilethe energy harvesting circuit is storing energy in its capacitor bank orbattery. The processor (and its integrated amplifier/RF stage) does notcome out of sleep mode until the energy harvesting circuit detects thatthe stored charge is adequate for the programmed task, such as reading asensor. This prevents the measurement system and processor from loadingthe energy harvesting circuit and allows storage of energy to proceedmost efficiently.

Further reductions in power consumption may be realized by using lowerclock rates for the embedded processor. For example our existingStrainlink digital wireless sensor transmitter product(http://www.microstrain.com/slink.html) is normally run at a clock rateof 4 MHz, and it consumes 600 microamps at 3 volts DC (1.8 milliwatts).For temperature measurement applications (or any other applicationrequiring relatively infrequent data samples), we can reduce theprocessor's clock rate to 100 KHz, allowing a more than 20 fold powerreduction to 28 microamps at 3 volts DC (0.084 milliwatts). For manyhealth monitoring applications, we can improve performance by simplyslowing down the system clock.

The energy required to power sensors, acquire data, and process/storethese data is much lower than the energy required to wirelessly transmitthese data. In the preceding discussion, we have shown thatthermocouples (0 milliwatts) with cold junction compensation (0.06milliwatts) could be combined with a smart microcontroller (0.084milliwatts) and a very low power, highly integrated signal conditioner(0.6 milliwatts) to provide continuous thermocouple readings with apower drain of 0.744 milliwatts. This is in sharp contrast to the RFcommunications section of the electronics, which may require over 10milliamps at 3 volts DC for a power drain of 30 milliwatts in order toprovide adequate wireless range and good margin in electrically noisyenvironments.

By programming the processor to acquire and log sensed data and tocompare these data to programmable threshold levels the frequency of RFtransmission can be reduced to save power. If the sensed data exceeds orfalls below the acceptable operating temperature ranges, then theprocessor transmits its data, along with its address byte. Arandomization timer is be used to insure that if multiple transmittersare transmitting their data (or alarm status) simultaneously, theprobability of RF collisions is statistically small, as described inhttp://www.microstrain.com/WWSN.html, a paper entitled SPIE ScalableWireless Web Sensor Networks, SPIE Smart Structures and Materials,March, 2002, by Townsend et al. In the event that threshold levels arenot crossed, stored summary data, such as mean, maximum, minimum, andstandard deviation, are periodically transmitted over time intervals,such as hourly or daily. Transmission of processed data, such as thesetrends, and periodic transmission of this data saves more energy.

The processors may also be programmed to acquire bursts of data from avibrating structure using an accelerometer. These data may be processedusing average fast fourier transform (FFT) and power spectral density(PSD) analyses. The processed data would allow the RF link to transmitonly the fundamental vibration frequencies, which would greatly reducethe amount of data that is transmitted and thereby greatly reduce the“on-time of the RF link.

To further reduce power consumed by the energy harvesting sensing nodes,we could reduce the RF communications power levels at the expense ofrange. For some applications, it may be possible for Navy maintenancepersonnel to approach an area where shipboard monitoring nodes have beenplaced. The energy harvesting monitoring nodes may also include RFtransceivers, which would provide for bi-directional communications.Instead of only periodically transmitting sensed data, these nodes areprogrammed to periodically activate their integral receiver to detectthe presence of maintenance personnel over the wireless link. A handheldtransceiver carried by maintenance workers would query various nodes onthe network and collect their data into the handheld device. This wouldgreatly reduce the need for long range wireless data communications, andtherefore would allow for reduced RF power levels at the remote energyharvesting sensor nodes. Microstrain has developed a high speed datalogging transceiver product that could be adapted to this purpose(http://www.microstrain.com/DataLoggingTransceiver.html).

The vibrational energy harvesting unit is illustrated schematically inFIG. 1 a-1 d. It consists of circuit board 25 that is rigidly fixed tosome vibrating component, such as vibrating machine 19. Leaf spring 22is mounted to this base in a cantilever configuration. Proof mass 24 issuspended at the free end of the leaf spring, and this can be adjustedto more nearly provide a resonance frequency close the vibrationfrequency. One or more PZT elements 20 are bonded to the surfaces ofleaf-spring 22 such that when spring 22 deflects, PZT 20 will undergotensile/compressive strains and therefore be stimulated to generate anelectrical output suitable for input into energy harvesting circuit 18′,18″. To maximize the output of PZT 20, leaf spring 22 is preferablyconstructed using a “constant strain” profile, as shown in FIG. 1 c,such that the strains experienced by the PZT elements are uniform alongtheir length. To provide a constant strain profile, leaf spring flexureelement 22 can have a taper, as shown in FIG. 1 c. Enclosure 26surrounds the device to keep contaminants out, and to make the deviceconvenient to handle and damage resistant.

Enclosure 26 measures approximately 50×50×150 mm and leaf spring flexureelement 22 has adjustable proof mass 24 of between 100 and 500 grams.Tuning the unit will be accomplished by adjusting the size of proof mass24, which can be washers, as shown in FIG. 1 c. The energy harvester iscapable of generating sufficient energy to intermittently power atransmitter and several low power sensors, as shown in FIGS. 3 a, 3 b,11 a, 11 b.

While several embodiments of the invention, together with modificationsthereof, have been described in detail herein and illustrated in theaccompanying drawings, it will be evident that various furthermodifications are possible without departing from the scope of theinvention. Nothing in the above specification is intended to limit theinvention more narrowly than the appended claims. The examples given areintended only to be illustrative rather than exclusive.

1-75. (canceled)
 76. An energy harvesting system, comprising: a wheel; aPZT mounted on said wheel; a circuit for harvesting energy from saidPZT, wherein said circuit comprises an element having an impedanceapproximately matching impedance of said PZT; and a sensing moduleconnected to said circuit, said sensing module including a sensor and acircuit for wirelessly transmitting sensor information.
 77. A system asrecited in claim 76, further comprising a tire mounted on said wheel,wherein said PZT is mounted on said tire.
 78. A system as recited inclaim 76, wherein said sensing module includes a sensor for sensing atleast one from the group including tire temperature and tire pressure.79. A system as recited in claim 76, further comprising a rechargeablepower supply.
 80. A system as recited in claim 76, further comprising amicro-controller.
 81. A system as recited in claim 76, furthercomprising a reactive device having a high impedance approximatelymatching impedance of said PZT at its operating frequency forefficiently transferring electrical energy from said PZT to saidreactive device.
 82. A system as recited in claim 81, further comprisinga low impedance high capacity storage device.
 83. A system as recited inclaim 82, further comprising a circuit for monitoring voltage acrosssaid reactive device and for transferring said electrical energy fromsaid reactive device to said low impedance high capacity storage devicewhen said voltage across said reactive device reaches a specifiedvoltage value.
 84. A system as recited in claim 76, wherein saidpiezoelectric transducer is connected to said storage device through arectifier.
 85. A system as recited in claim 84, further comprising acircuit having an impedance approximately matching load impedance of thePZT to improve efficiency of energy collection.
 86. A system as recitedin claim 85, wherein said circuit comprises a small capacitor and aDC-DC converter to transfer charge from said small capacitor to saidstorage device.
 87. A system as recited in claim 86, wherein saidcircuit further comprises a second switch connected to a terminal ofsaid small capacitor, wherein said second switch enables said DC-DCconverter when voltage across said small capacitor reaches a thirdthreshold value.
 88. A system as recited in claim 87, wherein said thirdthreshold value is a multiple of a turn on voltage of said secondswitch.
 89. A system as recited in claim 88, wherein said multiple isprovided by a voltage divider.
 90. A system as recited in claim 89,wherein said voltage divider has a total resistance exceeding 10megohms.
 91. A system as recited in claim 87, further comprising atiming capacitor for providing an RC time delay to keep said secondswitch on while said small capacitor discharges.